I. Field of the Invention
This invention relates to the use of new catalysts to accelerate the reaction of phosphorus pentasulfide with alcohols or phenols.
II. Description of the Prior Art
Phosphorus pentasulfide is widely used as a raw material in the manufacture of O,O-dialkyl- or O,O-diaryldithiophosphoric acids, which have many uses; e.g. in the formulation of anti-oxidation and anticorrosion "dopes" for motor oils, as ore flotation agents or as intermediaries for the synthesis of phosphorus insecticides.
The alcoholysis and phenolysis of phosphorus pentasulfide are generally described by the following reaction: EQU P.sub.2 S.sub.5 + 4 ROH .fwdarw. 2 (RO).sub.2 P(S)SH + H.sub.2 S.uparw.I
actually, as various authors have shown (K. Moedritzer and J. R. van Wazer, J. Inorg. Nucl. Chem. 25 (1963), 683-690; A. E. Lippman, J. Org. Chem. 31 (1966), 471-473; L. Nebbia and V. Bellotti, La Chimica e l'Industria 52 (1970)(4), 369-371), the main product (RO).sub.2 P(S)SH is always accompanied by minor quantities of other phosphorus esters: for example, (RO).sub.2 P(S)H, (RO).sub.2 P(S)SR, (RO).sub.3 PS, (RO).sub.2 POSH, RO--PS(OH).sub.2, RO--PS(SH)(OH), (RO).sub.2 P(S)--S--(S)P(OR).sub.2, (RO).sub.2 P(S)--SS--(S)P(OR).sub.2 even when purified phosphorus pentasulfide is used.
In addition, separation of a small quantity of sulfur can be observed, especially with commercial phosphorus pentasulfides, even when elementary analysis of the latter does not indicate the presence of sulfur in stiochiometric excess.
The speed with which an alcohol or a phenol reacts with phosphorus pentasulfide under given conditions may vary over a wide range depending upon the origin of the latter. The speed of this reaction is of great commercial importance because of its effect of the efficiency of the process plants in which dialkyl- and diaryldithiophosphoric acids are produced. The rate of this reaction is described by a "reactivity" index which is evaluated by means of a calorimetric alcoholysis test, the alcohol used usually being isopropanol. This index varies little with speed of agitation or the mesh size of the powdered P.sub.2 S.sub.5 reagent, but does depend greatly on the crystalline structure of the phosphorus pentasulfide.
According to P. Bencze (Revue de l'Institut Francais du Petrole XXV (1970), No. 5, 647-676), the alcoholysis of phosphorus pentasulfide takes place according to the following process:
(1) Rapid physical dissolution of the P.sub.2 S.sub.5 in the alcohol, followed by, PA1 (2) Slower chemical reaction of the dissolved P.sub.2 S.sub.5 with the alcohol.
The first step, being very rapid, tends in effect to keep the liquid alcohol saturated with P.sub.2 S.sub.5. The over-all speed of the reaction, as governed by the slow step (2), may be represented by the equation. ##EQU1## where (ROH) represents the alcohol concentration (which decreases with time) and (P.sub.2 S.sub.5) represents the concentration of dissolved P.sub.2 S.sub.5 (which is substantially constant). For a particular sample of P.sub.2 S.sub.5, Bencze determined the value of (P.sub.2 S.sub.5) in isobutanol at 40.degree. to be 0.15%.
Analysis of commercial phosphorus pentasulfides by X-ray diffraction shows that they generally consist of a mixture of an "abnormal" (amorphous or microcrystalline) phase and a normal, well crystallized, phase. The former is metastable and more soluble in solvents than the latter. According to Formula II, therefore, the metastable variety reacts faster with alcohols than the crystalline variety. The reactivity of a phosphorus pentasulfide sample will thus be in direct proportion to the amount of abnormal phase material present.
This "abnormal" phase, the structure of which has not been established, is perhaps identical with the highly active variety isolated by H. Vincent (thesis defended Feb. 19, 1969, before the Faculty of Sciences of the University of Lyon, France) via vacuum sublimation of P.sub.2 S.sub.5 heated to 200.degree.-220.degree. with condensation of the vapors on a wall cooled with liquid air.
It is also well known that commercial phosphorus pentasulfides when subjected to abrupt cooling or "quenching" from the molten state contain more abnormal phase and are more reactive than those allowed to cool slowly (see K. Moedritzer and J. R. van Wazer, op. cit. supra).
U.S. Pat. No. 3,023,086 states that the time taken to traverse the interval from 280.degree. to 260.degree. C. (pure P.sub.2 S.sub.5 melts at 288.degree.) is critical. The reactivity increases for example from 1 to 10 when this period is reduced from 2 min. 30 sec. to 0.125 sec.
However, such a quenching operation presents formidable technical problems on a commercial scale. U.S. Pat. No. 3,282,653 for example describes a conveyor belt system having three temperature zones serving to cool the phosphorus sulfide sequentially from 400.degree. to 250.degree. C.; development of such equipment on a commercial scale would seem a rather difficult matter. Other means have been proposed and tried with more or less success.
Another shortcoming of the quenching method consists in the metastable character of the pentasulfide produced, since its reactivity tends to decrease in storage or on contact with solvent vapors, or even in response to a temperature rise.
Furthermore, the quenching method requires a phosphorus pentasulfide user, who at times needs a high reactivity P.sub.2 S.sub.5 and at other times a lower reactivity P.sub.2 S.sub.5, depending on the alcohol or phenol which he may be using, to stock at least two different types to meet his requirements.
One way to avoid these disadvantages, is to increase the speed of reaction of the phosphorus pentasulfide by the use of catalysts. Whereas the quenching process increases the over-all reaction speed by modifying the structure of the P.sub.2 S.sub.5, the catalytic process apparently leaves this unchanged and acts on the reaction speed of the dissolved P.sub.2 S.sub.5, or in other words increases the value of the reaction constant k.
Unfortunately, the catalysts proposed up to now are few in number and not very effective unless used in excessive amounts.
U.K. Pat. No. 1,228,528 claims, for example, to employ traces of ammonia to catalyze the reaction of P.sub.2 S.sub.5 with alcohols and phenols.
P. Bencze (cited above) uses potassium phenate in a molar dose of 0.3% to activate the phenolysis of phosphorus pentasulfide.
N. I. Zemlyanskii and L. V. Glushkova (Zhurnal Obshchei Khimii 37(4) (1967), 775-777) react P.sub.2 S.sub.5 with 2,4-dichlorophenol and with 2,4,6-trichlorophenol in the presence of a massive quantity of triethylamine (2 mols to 1 of P.sub.2 S.sub.5).
N. I. Zemlyanskii and I. V. Murav'ev (Doklady Akademii Nauk SSR 163, 1965, No. 3,654-655) likewise use large amounts of triethylamine or potash to catalyze the reaction of P.sub.2 S.sub.5 with methyl and hexyl alcohols, phenol and p-nitrophenol.
Kalashnikov, V. P. (Zhurnal Obshchei Khimii 40 (1970), No. 9, 1954-1966), following the preceding method, react phosphorus pentasulfide with pyrocatechol in the presence of a stoichiometric quantity of triethylamine.
Lastly, M. G. Imaev, I. V. Tikunova and I. S. Akhmetzhanov (USSR Pat. No. 285,146) describe the reaction of P.sub.2 S.sub.5 with a C.sub.4 to C.sub.9 alcohol in the presence of azo-bis-isobutyronitrile.